This invention relates to the process for the removal of ammonia, hydrogen sulfide, and hydrogen cyanide from gases, especially coke oven gas, which is washed with an aqueous solution of metallic salts of sulfuric acid or sulfurous acid. More particularly, the present invention relates to the subsequent treatment of the washing solution in a closed system including the use of an oxidizer to remove sulfur in the forms of flowers of sulfur from the washing solution.
There are known processes utilizing an aqueous solution of caustic alkali, particularly iron sulfate, as an absorption agent to remove ammonia, hydrogen sulfide and hydrogen cyanide from gases. In these processes, the ammonia is changed into ammonium sulfate (NH.sub.4).sub.2 SO.sub.4. Most of the hydrogen sulfide is transformed into iron sulfide, FeS, and the hydrogen cyanide is transformed into iron cyanide compounds. Most of these processes provide for the subsequent treatment of the washing solution by processing it through an oxidizer wherein oxygen or gases containing oxygen are injected into an oxidizing vessel. In the oxidation process, elementary sulfur is liberated from the iron sulfide and removed in the form of flowers of sulfur and the resulting iron hydroxide is once again returned in the cycle for further washing of gases.
All of these known processes have the acute disadvantage that they require the constant addition of iron sulfate for the formation of ammonium sulfate and iron cyanide compounds and then these processes require the removal of the ammonium sulfate and the iron compounds from the cycle. This means, of course, that the employment of these processes is greatly limited by the fact that even if caustic alkali is readily and economically available, the production of crystalline ammonium sulfate is no longer economical from the standpoint of a saleable product since the need for products containing ammonia has been replaced by more favorable and economical processes. Furthermore, new environmental regulations require the total elimination of poisonous cyanide compounds.
One prior proposal for the removal of ammonia, hydrogen sulfide and hydrogen cyanide from coke oven gas by washing with metal salts, provides in the process that the gas is washed with an aqueous solution of metal salts of sulfurous acid and, if necessary, with the further addition of salts of sulfuric acid so that the gas bearing washing solution is carried in a cycle through an oxidizer. A part of the washing fluid leaving the oxidizer is diverted and, after the addition of a heating agent, subjected to a combustion process. The combustion products produced contain nitrogen, water vapor, a formed acid anhydride and metal or metallic oxide. The combustion products are then cooled. The acid anhydride and the metal or metallic oxide are reactively combined for the production of fresh salt solution which is then employed to maintain the aqueous metallic salt solution in the washing cycle. It is a necessary part of this process which is not part of the present level of technique, to employ special installations for the combustion process in order to produce fine divided or granulation of metal oxides from the combustion process. The most favorable grain size of these metal oxides was determined to be at a range between 5 and 20 microns. It is critically necessary that these metal oxides have such a fine granular size in order to achieve a reaction within a maintained period of time. This reaction takes place between the metallic oxide and the acid anhydride which is not economically favorable but acceptable in the process. This process has other significant limitations in that the specific capacity and maximum size of the combustion vessels are limited due to the dust-load factor, i.e., the dust density of the resultant combustion gases. In instances requiring a very large through-put capacity to the combustion process, then this could only be accomplished by providing and operating several installations for the combustion process in parallel with one another.